The US Supreme Court has rejected appeals against gay marriage in five states - Indiana, Utah, Oklahoma, Virginia and Wisconsin.
The move increases the number of states where same-sex marriage is legal to 30, plus the District of Columbia.
By declining to hear the appeals, the court left intact lower court rulings that had struck down those state bans.
Support for gay marriage in the US has gained momentum since the Supreme Court delivered two landmark rulings in 2013.
Monday's ruling means that the nine justices of the court have stopped short from resolving the question of same-sex marriage nationwide.
But it was received rapturously by gay marriage campaigners.
"Today is a joyous day for thousands of couples across America who will immediately feel the impact of today's Supreme Court action," said Chad Griffin, president of Human Rights Campaign.
But he urged the court to tackle "a complex and discriminatory patchwork of marriage laws" that remain in place.
"The only acceptable solution is nationwide marriage equality and we recommit to ourselves to securing that ultimate victory as soon as possible," he said.
US media reaction
"The Court's denial of review in all the pending cases strikes me as grossly irresponsible, as a huge abdication of duty on the part of at least six justices." - Ed Whelan, in the National Review
"Supreme Court declining these cases is a victory for gay marriage, but also for letting states do the lifting. Seems about right to me." - tweeted Jason Kuznicki of the Cato Institute
"The real question is what Justice Anthony Kennedy thinks. Is he ready and willing to hold that the Constitution guarantees a right to same-sex marriage? Or is he still conflicted given his federalist sympathies?" - Jonathan H Adler, writing in the Washington Post
The ruling immediately lifted the delay on same-sex weddings in the five states, and within hours there were wedding bells in Virginia.
Thirty-year-old Lindsey Oliver and 42-year-old Nicole Pries received the first same-sex marriage licence issued from the Richmond Circuit Court Clerk's office shortly after 13:00 local time.
Couples in six other states - Colorado, Kansas, North Carolina, South Carolina, West Virginia and Wyoming - are expected to be able to get married soon.
Last year, the Supreme Court struck down a federal law that barred the federal government from recognising same-sex marriage.
That led to several victories across the US as gay marriage bans were struck down in lower courts.
Many expected the Supreme Court to use this session to, at some point, tackle the issue nationwide, rather than address it state-by-state.